Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2000-01-19
2002-04-02
Rickman, Holly C. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S690000
Reexamination Certificate
active
06365287
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetic recording medium suitable for high-density recording and also to a magnetic storage device with said magnetic recording medium.
The magnetic recording system is divided into two types, that is, longitudinal recording and perpendicular recording, with the former being widely prevalent. The longitudinal magnetic recording system carries out magnetic recording by forming recording bits by the magnetic field generated by the magnetic head in such a way that the N-pole of one bit butts against the N-pole of its adjacent bit and the S-pole of one bit butts against the S-pole of its adjacent bit, the recording bits being arranged parallel to the plane of the magnetic recording medium. For this recording system to have a high recording density and to generate a high reproducing output, it is essential to reduce the effect of demagnetizing field on the recorded bits. To this end, attempts are being made to reduce the thickness of the magnetic film and to increase the coercive force in the magnetic film.
The perpendicular recording system performs magnetic recording in the following way. Recording bits are formed by the magnetic field of the magnetic head in the direction perpendicular to the film plane of the magnetic recording medium having the perpendicular magnetizing anisotropy, with adjacent bits being magnetized in the anti-parallel direction. Thus, the magnetic pole of one bit has a polarity opposite to that of its adjacent bit. As a result, the magnetic moments of adjacent bits attract each other. This stabilizes magnetization for recording and increases the coercive force, thereby contributing to high-density recording.
In both recording systems, an increase in coercive force is an important factor to improve the recording density. One of the factors to determine coercive force is magnetocrystalline anisotropy energy. This is a measure to indicate the ease with which the magnetic moment in magnetic crystal grains is oriented in a specific crystalline direction. The greater the value, the easier the orientation. For example, in the case of Co crystal grains, the magnetic moment easily orients in the direction of the c axis of the hexagonal closed-pack crystal lattice. (This is the axis of easy magnetization.) The magnetocrystalline anisotropy energy (or the magnetic anisotropy constant) K
u
is 4.6×10
6
erg/cm
3
.
The energy to orient the magnetic moment in crystal grain in the direction of axis of easy magnetization is given by K
u
V, where V is the volume of crystal grain. On the other hand, the magnetic moment fluctuates due to thermal vibration. The energy of thermal vibration is given by k
B
T, where k
B
is Boltzmann constant and T is an absolute temperature.
The behavior of the magnetic moment varies depending on the relative magnitude of k
B
T and K
u
V. If k
B
T<<K
u
V, the magnetic anisotropy energy is sufficiently large and hence the magnetic moment orients approximately in the direction of the c axis of crystal grain. If k
B
T>>K
u
V, the energy of thermal vibration is larger than the magnetic anisotropy energy and hence the magnetic moment continues thermal vibration (super paramagnetic state). This thermal vibration causes the inversion of magnetic moment to take place with a certain probability per unit time. For example, the energy of thermal vibration required for the inversion of magnetic moment to take place with a probability of 1/e per second is 25 k
B
T. If this inversion takes place, the coercive force decreases as time lapses along with the probability, resulting in a decrease in recording density. Therefore, the recording medium should at least meet the condition of 25 k
B
T<<K
u
V.
In the meantime, among the related art media for high-density magnetic recording is magnetic film of Co
81
Cr
15
Ta
4
alloy. (See IEEE Transaction of Magnetics, vol. 34, No. 4 (July 1998), pp. 1558-1560, as the first US literature.) This magnetic recording medium has a magnetic anisotropy energy K
u
of 1.3×10
6
erg per cm
3
at about 300 K (absolute temperature T).
The above-mentioned medium is characterized by a magnetic grain size of about 15 nm (on average) and a film thickness of about 20 nm. The magnetic anisotropy energy possessed by a single magnetic crystal grain is K
u
V=4.6×10
−12
erg. On the other hand, the energy of thermal vibration at room temperature (300 K) is k
B
T=4.1×10
−14
erg. Thus, K
u
V>>25 k
B
T. In other words, under the present condition of crystal grain size, the magnetic anisotropy energy is much larger than the energy of thermal vibration, and hence the magnetic moment is fixed in the direction of axis of easy magnetization and this leads to a sufficiently large coercive force.
For both recording systems to have an increased recording density, it is important not only to increase the reproduction output for high-density recording but also to reduce the noise of the medium. The noise of the medium in a state of high-density recording results from the zigzag magnetic domain wall in the transition region of the recording bit. The greater the fluctuation of the magnetic domain wall, the greater the noise. Thus, common practice to decrease noise is to reduce the particle size of the magnetic crystal grains constituting the magnetic recording medium, thereby to reduce the fluctuation of the magnetic domain wall in the transition region.
The related art recording density (as experimental data) is 10 Gbit per square inch (as stated at the 7
th
MMM-Intermag Joint Conference (January 1998, Session ZA). This recording density corresponds to a linear recording density of about 400 kFCI (Flux Change per Inch, or magnetization reversal number per inch), with the bit length being about 60 nm, assuming that the ratio of bit length to track width is about 20:1, which is common.
SUMMARY OF THE INVENTION
The thin-film medium for longitudinal magnetic recording now in use has a crystal grain diameter of about 15 nm. This implies that only four crystal grains arranged in the bit direction currently constitute a bit which is 60 nm long. This results in a large zigzag magnetic domain wall in the transition region. In other words, the magnetic domain wall fluctuates so greatly as to give rise to a problem in noise.
If the related art medium described in the first literature given above is designed such that the crystal grain size is 10 nm and the film thickness is 10 nm according to the related art technique so as to raise recording density and reduce noise, then the magnetic anisotropy energy of the crystal grains will be K
u
V=1.2×10
−12
erg. This value is about one-forth of that possessed by the related art magnetic crystal grain before design. However, the relation of K
u
V>25 k
B
T is satisfied.
Unfortunately, simply reducing the crystal grain size and film thickness as mentioned above results in a medium which has a low coercive force at the higher operating temperature range (as mentioned later), and this medium does not produce sufficient high reproducing output. In other words, as the temperature of crystal grains rises by 50 K, reaching 350 K, the energy of thermal vibration increases to k
B
T=4.8×10
−14
erg. On the other hand, the magnetic anisotropy energy usually decreases with increasing temperature, and it disappears at the Curie point. In the case of Co
81
Cr
15
Ta
4
alloy described in the first US literature mentioned above, K
u
1.3×10
6
erg/cc at T=300 K, while K
u
=1.0×10
6
erg/cc at T=350 K. In other words, a temperature rise of 50 K causes K
u
to decrease by 20% or more.
Consequently, the magnetic anisotropy energy of a single crystal grain at T=350 K is K
u
V=7.9×10
−13
erg, and the relationship between the energy of thermal vibration and the magnetic anisotropy energy becomes K
u
V<25 k
B
T. The result is that the magnetic moment in a crystal grain is hardly fixed in the direction of axis
Igarashi Masukazu
Inaba Nobuyuki
Takahashi Yoshio
Takeuchi Teruaki
Yamamoto Tomoo
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
Rickman Holly C.
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